The separation between first and second points is determined. At the first point are first, second and third mutually orthogonal coils that are excited in sequence, so that first, second and third magnetic fields are sequentially derived. At the second point are first, second and third mutually orthogonal...http://www.google.de/patents/US4560930?utm_source=gb-gplus-sharePatent US4560930 - Distance-measuring system using orthogonal magnetic field generators and orthogonal magnetic field sensors
